Composition for and method of descaling metal parts



Patented July 1, 1952.

UNITED OOMPOSITEON FOR AND METHOD OF DESCALING METAL PARTS Artemas F. Holden, New Haven, Conn.

Ne Drawing. Application July 2, 1947,

, Serial No. 758,720

parts by the use of one or more molten salt baths,

with or without an attendant change in the physical properties of the parts so treated, and to the molten salt baths used. The temperature at which the descaling is performed is varied dependent upon the chemical composition of the bath and of the parts to be treated, and whether mere descaling or descaling together with hardeningor annealing is desired.

' Thisapplication is a continuation in part of my application "Serial Number 500,645, filed August 31, 1943, which matured into Patent 2,431,- 479, dated November 25, 1947.

Metal parts, particularly ferrous parts, are subject to the formation of an oxide scale layer at elevated temperatures. This layer is often f ormed during an initial heat treatment or welding of such. parts and is usually tightly adherent to the parts. Scale must be removed from the parts if a bright finish is to be secured or if the parts are to be-subsequently plated. This layer of scale may be removed by various chemical processes such as pickling baths, which also attack the metal somewhat, or by the bright tempering salt bath disclosed in my copending application, Serial .Number 430,716, filed February 13, 1942, which matured into Patent 2,426,773, dated September 2, 1947. The present invention relates to. a descaling process and composition in which the initial or first bath is comprised entirely or principally of chlorides of the alkali and alkaline earth metals to materially alter the carbon content of various ferrous parts placed therein. The bath will, however, attack any metal oxide, paint or lacquer present on the. work without effecting the metal itself. The oxide scale is loosened by thisbath and principally becomes detachedfrom the work during a final water quench. The process is useful in the descaling of ferrous metals and nickel and copperandtheir alloys. 1 i Theifirst salt bath ofthis iprocess, operable between 1100 F. and v1600 =F.,may comprise scda ash 45%, potassium chloride"44%, and sodium chloride l=l%,,tow hich is added 2% to "%of fluoride of sodium, potassium or barium and /2 to 5% silicon carbidapreferably in the amount 16 Claims. (01(134-29) Another first bath to which the fluorideand silicon carbide may be added to come within above proportions of these chemicals is comprised of potassium chloride 42 sodium chloride 55 /2 and sodium fluoride 2 the initial sodium fluoride percentage being raised by additional amounts of this compound. This bath. is operable between 13'00F. and 1700 F.

Another first bath may consist of potassium chloride 30%, sodium chloride 25%, soda ash 35 /2% and sodium fluoride 9 727, to which is added silicon carbide within the range set forth abovev This bath is operable between 1300" F. and 1700 F.

Another first bath may be formed of sodium chloride ni calcium chloride 21 /29 0 potassium chloride 25% and barium chloride 39%, to which is addedthe fluoride and silicon carbide in the ranges above set forth. This bath is operable between 1050 F. and 1500 F.

A still further first bath may be comprised of barium chloride 78%, sodium chloride 56 K270 and calcium chloride Mi /2%, to which is added silicon carbide within the range above specified. This bath is operable between 1450" F. and .2000" F.

The first bathis maintained in molten state at a fixed temperature, preferably in an electric mersed' inthe bath until it attains the temperature of the bath and untilthe oxide scale has been attacked by the bath, which may be as long as half an hour. It is thenremoved from the bath and immersed in a second bath withoutsubstantial loss of temperature tothe metal part. The effect of theinclusion of any'of the fluoride compounds above mentioned together with silicon carbide to any of the baths above .set forth is to react with any scale on the surfaceof themetal part. This action is probably -principally caused by action of-the fluoride although this action is enhanced by the silicon carbide The action of the silicon carbide is believed to be largely in the production of carbon monoxide small quantities on the surface of the metal which tends to mechanically loosen the scale.

A further and perhaps principal descaling effect of silicon carbide in conjunction with the fluoride content of the bath is, without doubt, similar to the use of chlorides as discussed in Patent No. 2,188,063, issued to Haig Solakian on January 23, 1940, for a case hardening composition. In this carburizing or case hardening bath the fluorides act as a stabilizer to reduce the rate at which sodium cyanide is oxidized in reaction with air. Inasmuch as the fluoride acts as a stabilizer for carbon monoxide and nitrogen, it is presented that it acts in the same manner for stabilizing silicon carbide upon its release of carbon monoxide, in conjunction with the other elements of the bath which react to promote reduction of scale.

It has been found that when two media of rectification are used, namely, boric acid in conjunction with silicon carbide, the effect of boric acidis slight at high temperatures, whereas silicon carbide may be added in any quantity within reason regardless of temperature. The probable conclusion as to the lesser effect of boric acid at higher temperatures in relation to the effect of silicon carbide at such temperatures is possibly higher absorption properties of the annealing or heat treating bath and greater assimilation of metallic oxides than at lower temperatures.

' Silicon carbide presumably is as heavy and probably heavier than metallic oxides encountered in the bath. Sodium silicate formed by the reaction of silicon carbide with alkali reaction products is heavier than the bath and tends to settle to the bottom, cleaning the bath. This and the above or basic suppositions cover past experience which, however, cannot be conclusively checked by actual chemical test of the sludge formed in the bottom of the bath or by changes in melting point of the original compound. I Primarily, there seems to be a combined efiect of the fluoride and the silicon carbide, and probably some of this combined effect of the materials is reactive on the surface of the metals before quenching.

The second salt bath used in this descaling process comprises at least 60% zinc chloride together with alkali and alkaline earth metal chloride, which is operated at a temperature between 900 F. and 1100 F. The temperature of this bath is below the critical temperature of the ferrous parts immersed therein and the bath may be used at that temperature as a quenching bath on austenitic stainless steels which are descaled at the high temperature.

Where work is to be descaled at the normalizing or annealing temperature, if the work is first introduced into an intermediate salt bath at a temperature from 1450 to 1700", it is then transferred into-abath of the same chemical composition at the critical point of the steel and later transferred to the zinc chloride composition bath operated between 900 F. and 1100 F.

This bath is particularly important when the work is to be descaled and annealed at the critiquenched in water.

The parts are usually retained in the various component baths for definite intervals. For example, in the 1350 F. or high temperature bath, from five to thirty minutes are required. The same time cycle may be used in the bath operating at from 900 F. to 1100 F.

After the parts have been held in the bath at 900 F. to 1100 F. for the standard interval of time, they are withdrawn and immediately This water quench removes the scale by explosion of the salt and whatever residue of scale is left behind, and produces a bright finish on the work so processed. Removal of the scale in the final quench is accomplished by the reduction of the scale in the high temperature and quenching baths, the scale having been already loosened or removed in part in these baths before the water quench so that the steam formed on the surface of the metal during the water quench forces the scale from the metal, thus completing the cleaning operation.

The use of small quantities of boric acid in conjunction with the silicon carbide is to aid in settling out the metallic oxides by formation of a sludge. In general, boric acid is not considered to be of aid in the descaling operation except in the settling out of metallic oxides present which tend to contaminate the bath. This sludge which settles to the bottom of the pot may be removed therefrom by a suitable sludge ladle, so that there will be no possibility of reassimilation by the bath of any of this material.

It is recognized that when high temperature baths are used and parts therefrom are transferred to lower temperature baths of a different chemical composition, the lower temperature quenching bath is contaminated by the material of the first or high temperature bath. Accordingly, for the present series of baths, pure zinc chloride may be added to the bath operating at 900 F. to 1100 F. to maintain sufiicient fluidity of the bath at the operating temperature and maintain its chemical composition more or less uniform. Inasmuch as the specific gravity of zinc chloride is heavier than potassium or sodium chloride, excess dragout of these latter salts-from the lower temperature bath results, thus aiding in obtaining a stable operation. 7

Under certain conditions, the quenching bath which operates at 900 F. to 1100 F. may be eliminated, particularly where parts are to be pickled or passivated, such as for various types of stainless steel which are pickled in a 20% nitric acid solution at F. When the 900 F. to 1100 F. bath is eliminated, the work has some dark stains or smut which is a reduced metallic oxide. The principle, however, of the operation and selection of temperature of the baths utilizing silicon carbide with alkali or alkalineearth metal fiuoride in combination as a rectifier to produce small quantities of carbon monoxide, provides an effective method of simultaneously annealing'and descaling asoutlined above.

What I claim is:

1. A work descaling salt bath'adapte'd' to be operated at a temperature'above 1000 F. consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, together with 2% to 10% of afluoride selected from the group consisting of the fluorides ofsodium, potassium and barium, and /2% to 5% of silicon carbide.

.2. A work descaling salt bath adaptedto be operated at a temperature above 1000* 1 consisting essentially of a mixture of chlorides from the ast ma,

group consisting of alkali metal chlorides and alkalineearth metal chlorides, soda ash, 2% to of a fluoride selected from the group con sisting' of the fluorides of sodium, potassium and barium, and to 5% silicon carbide, said chlorides mixture comprising the principal portion of the bath.

3. A work descaling salt bath adapted to be operated at a temperature above 1000 F. consisting'essentiallyof a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, soda ash, 5% sodium fluoride and to 5% silicon carbide, said chlorides mixture comprising the principal portion of the bath.

4 A work descaling salt bath adapted to be operated at a temperature above 1000 F; consisting essentially of sodium chloride, potassium chloride, soda ash, 5% sodium fluoride and /z% to 5% silicon carbide, the sodium chloride and potassium chloride together comprising-the principal portion of the bath.

I 5. The method of descaling metal parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, 2% to 10% of a fluoride selectedfrom the group consisting of the fluorides 'of sodium, potassium and barium. to which silicon carbide is added from time to time to maintain to 5% of silicon carbide in the bath, and operated at a temperature above 1000 F., the parts being retained in the bath until heated to the temperature of the bath; the parts then being removed from the b'ath'and quenched in Water.

6. The method of descaling metal parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metalchlorides, 2% to 10% of a fluoride selected from the group consisting of the fluorides of "sodium, potassium and barium, and Az% "to 5% of silicon carbide and operated at a temperature above 1000 F., the parts being retained in the bath until heated to the temperature of the bath; the partsthen being removed from the bath and quenched iii-water.

7. The method of descaling metal-parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chloridesand alkaline earth metal chlorides, soda ash, 2% to 10% of a fluoride selected from the group consisting of the fluorides of sodium, potassium and barium, the chlorides mixture comprising the principal portion of the bath, to which silicon carbide is added from time to time to maintain to 5% of silicon carbide in the bath, and operated at a temperature above 1000 LEE, the parts being retained in the bath until heated to the temperature of the bath; the parts then being removed from the bath and quenched in water.

8. The method of descaling metal parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides and substantially 5% sodium fluoride, to which silicon carbide is added from time to time to maintain substantially 2% silicon carbide in the bath, and operated at a temperature above 1000' F., the parts being retained in the bath until heated to the temperature of the bath; the parts then being removed from the bath and quenched in water.

"9. Themethod ofdescaling metal parts comprising immersing them ina molten bath consisting essentiallyof a mixture of chlorides from the group consisting af alkali metal chlorides and alkaline earth metal chlorides, substantially 5% sodium'fluoride and substantially 2% silicon carbide and operatedat a temperature above 1000 R, the parts being retained in the bath until heated to the temperature of the bath; the parts then being removed from the bath and quenched in water. 10. The method of descaling metal parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chloridesjsoda ash, substantially 5% sodium fluoride, the chlorides mixture comprising the principal portion of the bath, to

Which'silicon carbide is added from time to time to maintain substantially 2% silicon carbide in thebath; and operated at a temperature above 1000 F., the parts being retained in the bath until heated to the temperature of the bath; the parts then being removed from the bath and quenched in Water.

11. The method of descaling metal parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, 2% to 10% of a fluoride selected from the group consisting of the fluorides of sodium, potassium and barium, and to 5% of silicon carbide, and operated at a temperature above 1000 F., the parts being'retained in the bath until heated to the temperature of the bath; the parts then being transferred to a second molten salt bath without substantial loss of temperature to the transferred parts, the second bath being operated at a temperature below the temperature of the first bath and between 1100 F. and 900 F., the second bath consisting essentially of a mixture of chlorides fromthe group consisting of alkali metal chlorides and alkaline earth metal chlorides, togethe'r withat least 60% zinc chloride, the parts from the group consistingof alkali metal chlorides and alkaline earth metal chlorides and2% to 10% of afiuoride selected from the group consisting of the fluorides of sodium, potassium or barium to which silicon carbide is added from time to time to maintain to 5% of silicon carbide in the bath, and operated at a temperature above 1000 F., the parts being retained in the bath at least until heated to the temperature of the bath; the parts then being transferred to a second molten salt bath without substantial loss of temperature to the parts therein transferred, the second bath being operated at a temperature below the temperature of the first bath and between 1100 F. and 900 the second bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, together with at least 60% zinc chloride, the parts being retained in the second bath until they have acquired the temperature of said bath; the parts then being removed from the second bath and quenched in water.

13. The method of descaling metal parts comprising immersing them in a molten salt bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, soda ash and 2% to 10% of a fluoride selected from the group consisting of the fluorides of sodium, potassium and barium, the chlorides mixture comprising the principal portion of the bath, to which silicon carbide is added from time to time to maintain to 5% of silicon carbide in the bath, and operated at a temperature above 1000 F., the parts being retained in the bath until heated to the tempertaure of the bath; the parts then being transferred to a second molten salt bath without substantial loss of temperature to the parts therein transferred, the second bath being operated at a temperature below the temperature of the first bath and in excess of 900 F., the second bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, together with at least 60% zinc chloride, the parts being retained in the second bath until they have acquired the temperature of said bath; the parts then being removed vfrom the second bath and quenched in water.

14. The method of descaling metal parts comprising immersing them in a molten bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, substantially 5% sodium fluoride and substantially 2% silicon carbide, and operated at a temperature above 1000 F,. the parts being retained in the bath until heated to the temperature of the bath; the parts then being transferred to a second molten salt bath Without substantial loss of temperature to the transferred parts, the second bath being operated at a temperature below the temperature of the first bath and between 1100 F. and 900? F., the second bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, together with at least 60% zinc chloride, the parts being retained inthe second bath until they have acquired the temperature of said bath; the parts then being removed from the second bath and quenched in water.

15. The method of descaling metal parts comprising immersing them in a molten salt bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides and substantially 5% sodium fluoride to which silicon carbide is added from time to time to maintain substantially 2% silicon carbide in the bath, and operated at a temperature above 1000 F., the parts being retained in the bath at least until heated to the temperature of the bath; the parts then being transferred to a second molten salt bath without substantial loss of temperature to the parts therein transferred, the second bath being operated at a temperature below the temperature' of the first bath and between 1100 F. and 900 the second bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, together with at least 60% zinc chloride, the parts being retained in the second bath until they have acquired the temperature of said bath; the, parts then being removed from the second bath and quenched in water.

16. The method of descaling metal parts comprising immersing them in a molten salt bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, soda ash and substantially 5% sodium fluoride, the chlorides mixture comprising the principal portion of the bath, to which silicon carbide is added from time to time to maintain substantially 2% silicon carbide in the bath, and operated at a temperature above 1000 F., the parts. being retained in the bath until heated to the temperature of the bath; the parts then being transferred to a second molten salt bath without substantial loss of temperature to the parts therein transferred, the second bath being operated at a temperature below the temperature of the first bath and in excess of 900 F., the second bath consisting essentially of a mixture of chlorides from the group consisting of alkali metal chlorides and alkaline earth'metal chlorides, together with at least 60% zinc chloride, the parts being retained in the second bath until they have acquired the temperature of said'bath; the parts then being removed from the second bath and quenched in water.

' 'ARTEMAS F. HOLDEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. A WORK DESCALING SALT BATH ADAPTED TO BE OPERATED AT A TEMPERATURE ABOVE 1000* F. CONSISTING ESSENTIALLY OF A MIXTURE OF CHLORIDES FROM THE GROUP CONSISTING OF ALKALI METAL CHLORIDES AND ALKALINE EARTH METAL CHLORIDES, TOGETHER WITH 2% TO 10% OF A FLUORIDE SELECTED FROM THE GROUP CONSISTING OF THE FLUORIDES OF SODIUM, POTASSIUM AND BARIUM, AND 1/2% TO 5% OF SILICON CARBIDE.
 5. THE METHOD OF DESCALING METAL PARTS COMPRISING IMMERSING THEM IN A MOLTEN BATH CONSISTING ESSENTIALLY OF A MIXTURE OF CHLORIDES FROM THE GROUP CONSISTING OF ALKALI METAL CHLORIDES AND ALKALINE EARTH METAL CHLORIDES, 2% TO 10% OF A FLUORIDE SELECTED FROM THE GROUP CONSISTING OF THE FLUORIDES OF SODIUM, POTASSIUM AND BARIUM TO WHICH SILICON CARBIDE IS ADDED FROM TIME TO TIME TO MAINTAIN 1/2% TO 5% OF SILICON CARBIDE IN THE BATH, AND OPERATED AT A TEMPERATURE ABOVE 1000* F., THE PARTS BEING RETAINED IN THE BATH UNTIL HEATED TO THE TEMPERATURE OF THE BATH; THE PARTS THEN BEING REMOVED FROM THE BATH AND QUENCHED IN WATER. 